This project is concerned with the early events in differentiation of B lymphocytes, with particular reference to the genetic events which are involved in expression of clonal and isotype diversity. Using an animal model in which B cell development is blocked at the level of the pre-B cell, we will study the expression of clonal diversity by this earliest member of the B cell lineage. We wish to determine whether pre-B cells in adult bone marrow contain a full spectrum of diverse clones, or whether they express only clonotypes characteristic of the neonate. In other studies we will confirm or refute earlier data indicating that individual B lymphocytes at a certain stage of their development simultaneously express three immunoglobulin isotypes. We will use techniques for culturing individual cells to study the phenomenon of isotype switching, in an attempt to determine the point at which cells become committed to expression of particular isotype(s). We will examine the relationship of ontogenetic acquisition of sIgD, Ia antigens, and C3 receptors to a critical change in the function of developing B lymphocytes through which they lose extreme susceptibility to tolerance induction. Finally, we will attempt to produce monoclonal anti-idiotypic antibodies using cell hybridization techniques, to be used in future studies of expression of clonal diversity during ontogeny.